Liquid anhydrous ammonia (NH.sub.3) has long been used as a fertilizer for corn, grains and other crops. When originally built, ammonia storage plants were typically located outside of rural towns, on rail sidings when possible. Now many of these towns have grown out to these plants, greatly increasing the potential exposure of town people to ill effects of ammonia in the event of a mishap at the plant.
The market for agricultural ammonia for a given ammonia storage facility is typically within only a 15-mile radius of the facility. The entire market area for any one facility thus becomes ready to receive ammonia at approximately the same time. The influence of weather greatly determines the length of the ammonia market season, but the season is typically crowded into only three months out of a year.
An empty nurse tank coming in from the field is first weighed, then filled, and then weighed again on its way back to the field. This operation typically requires a minimum of 20 minutes, and a typical 20-mile round trip would require another 25-30 minutes. A big tool bar in the corn country in good soil will apply close to a wagon of ammonia an hour. This leaves little time for the hauler between runs.
These conditions do not lend themselves to a well organized and safe operation.
There are three basic filling systems currently in use. The simplest system is a large high volume, high pressure pump that condenses vapor in nurse tanks eliminating the need of a vapor return connection to the storage tank. An intermediate system, in terms of complexity, is a pump with a vapor connection from the nurse tank to the storage tank. The third and most complex system currently used is a vapor transfer system, where an ammonia compressor transfers vapor from the nurse tank to the storage to move liquid through a separate line to the nurse tank.
When a nurse tank is pulled to a loading island, the operator chocks one wheel of the wagon, removes dust caps from nurse tank valves, connects filler hoses, opens them, opens tank valves, starts the pump or the compressor, and then opens a fixed level vent which gauges the proper filling level. Once the nurse tank is properly filled, the operator closes the hose end valves, closes the fixed level valve, closes the tank valves, vents the product (vapor) that remains between the tank valve and the hose end valve, which assures the pressure-tightness of the hose end and the tank valve, then he removes the hoses from the nurse tank valves, replaces the dust caps, and then removes the chock from the wagon wheel. The nurse tank is then ready to be weighed.
A recently-enacted safety requirement (CGA-G2.1 and ANSI-K61-1-5.10.8.1) provides that all stationary ammonia storage installations shall have an approved emergency shut off valve installed in the fixed piping of the transfer system within five lineal feet of where the hose or swivel piping is attached to the fixed piping. The emergency shut off valve shall be installed in the facility piping so that any break will occur on the side of the hose or swivel connection. The requirement also states that an approved emergency shut off valve shall incorporate a manually activated shut off from a remote location and a manually activated shut off at the installed location.
This new safety requirement reflects the fact that all too often a vehicle with a nurse tank in tow drives away from a loading island with the liquid ammonia filler hose still attached to the nurse tank. A "drive-away" is the most unfortunate mishap that can occur around an ammonia storage facility, with several potential results of varying severities. The least severe result is a break of the nurse tank filler valve just below the acme threads with both the hose line valve and the nurse tank valve closed--the most devastating result: a partial separation of the filler hose or partial separation of the plant piping with all valves open.
Most of the piping systems in use today are not securely anchored at the loading islands and should a "drive away" occur with the hoses connected and with the hose line and tank valves closed, the only separation of the system without product loss would be just below the male acme threads of the nurse tank valves--unfortunately this is not where the majority of separations occur. Because the loading island piping systems are not secure, it is doubtful that a shear point in the system without a solid anchor would remove all of the potential for disaster. The ideal system would have the facilities piping system well anchored at the filling island with the safety valve no more than six inches above the concrete mounting, with a predetermined separation point that would separate instantly and cleanly, and with means to close the system from both ends and the wagon traffic so directed that in the event of a drive away there would be a straight pull on the hoses.